The invention relates to a novel information recording medium and the recording of information thereon. More particularly, the invention relates to an optical recording medium, preferably in the form of a disk, or in tape format, suitable for use with an optical recording and playback apparatus, which recording medium is stable yet readily and accurately erasable.
It has long been apparent that optical recording, and in particular, optical disk recording, is a very promising recording method as it allows instantaneous playback, very fast random access and much higher recording density than is possible with magnetic recording and archival storage. It is widely recognized that the optical disc system with the greatest potential is the type that uses a highly focused laser beam as an ultra-fine recording stylus to store encoded information at very high data rates with extremely high density.
Optical recording methods have been proposed in which light from a laser is focused upon the surface of a recording medium with sufficient intensity to cause a detectable change in the physical characteristics of the surface material. Among these methods is the establishment of an information pattern of pits or bumps. In such methods, the representative information pattern of pits can be formed in the surface of the recording medium by suitably controlling the intensity of the focused light in accordance with the information to be recorded while relative motion is established between the recording medium and the focused light spot.
The recording process must have a sufficiently high resolution to allow the recording of bumps or pits of very small size to satisfy the requirement of high density, and the sensitivity of the process must be high enough to permit the use of economical, low power recording lasers. But the sensitivity should be low enough so that the recording medium can withstand a playback exposure high enough to yield a high signal-to-noise ratio. Furthermore, it is clear that in order to avoid long term degradation due to ambient conditions and to ensure totally non-destructive readout, the process must have a well defined exposure threshold below which there is no response. In addition, it is desirable that the recording process permit instant playback without the need for post-recording processing before readout. None of the presently known processes meet all these requirements.
In the past, there have been increasing efforts to develop an information recording method in which information is written in a thin film of metal or the like formed on a substrate by using a laser ray or beam. According to such a method, information recording has been accomplished by forming holes or recesses in metallic thin films under the action of a thermal energy beam such as a laser ray. See U.S. Pat. No. 4,238,803.
The sensitivity of the films for laser film deformation recording can be enhanced by the addition of pigments or dyes which exhibit a high absorption at the laser emission wavelength. Erasure of the film deformation is accomplished by recording over the information to be erased using a similar laser beam but with a much smaller scan line spacing, preferably to provide overlap of the scan lines.
U.S. Pat. No. 4,977,064 describes an optical recording medium of two recording media layers with each layer containing different light-sensitive dyes. The two layers have individual specific reflectance, transmittance and absorbance which allows for detection of a variation in the amount of light reflected from the boundary between the two layers.
In another recording system, the recording medium comprises two layers of first and second materials. The first layer of material is a relatively inextensible metal or polymer having a low thermal conductivity and a high coefficient of thermal expansion. The second layer of material possesses a glass transition temperature considerably above that of the bottom layer material and has low thermal conductivity. U.S. Pat. No. 4,371,954 discloses a dual layer medium with both layers being composed of metal. U.S. Pat. No. 4,719,615 describes the use of elastomers for the first layer and thermoplastic resins for the second layer. Other multiple layer film media imparting different optical and thermal mechanical properties are described in U.S. Pat. Nos. 4,901,304 and 4,825,430.
Characteristic of these multiple film media is the use of a retention layer and an expansion layer, which cooperate to form erasable, non-vesicular bumps projecting into a compression layer. The retention layer is necessary to maintain the bump, because of the instability of the expansion layer; the expansion layer will contract upon cooling. Upon exposure of the material with a focused laser beam, the beam is transmitted through the retention layer to the underlying expansion layer where most of the energy is absorbed in the expansion layer material, locally heating the expansion layer and causing it to expand to form a bump. This exposure also heats the retention layer to above its glass transition temperature causing it to melt. The swelling of the expansion layer deforms the melted retention layer, pushing it into the form of the bump, as well. After bump formation, the retention layer will cool below its glass transition temperature, thus fixing the shape of the bump as data stored in the medium. Over time, the underlying expansion layer will contract to substantially its original shape.
To erase, both layers are again heated to melt the retention layer and expand the expansion layer. The expansion layer is then cooled while the heated retention layer conforms to its shape. Ultimately, a cooled, flat expansion layer is obtained with a corresponding flat retention layer. The retention layer is then cooled to fix the shape.
In another type of optical recording system, a laser beam is focused on an erasable data storage medium to heat the medium and thereby induce a stable transition from one morphological state to another morphological state, such as the amorphous to the crystalline phase. The two physical states have different optical properties, i.e. the optical transmittance and optical reflectance properties of the respective states. Consequently, to read a data bit, light from a laser beam, which is at a lower power level than is used for writing, is focused on the data storage medium and will be reflected by the medium depending on the physical state of the medium. See U.S. Pat. No. 4,264,986.
However, the above materials change only slightly from one state to another and are not substantially distinguishable using optical means. For example, the optical data storage medium, disclosed in U.S. Pat. No. 4,975,355 includes semiconductor or chalcogenide materials. These materials yield very low signal-to-noise ratios due to insignificant changes in the recording medium. Furthermore, the stability of these materials is insufficient in that over time they will transform naturally from their amorphous state to their crystalline phase. Additionally, the data storage medium will physical fatigue after a relatively few number of erase/write cycles because of the delicate nature of the material utilized for the physical state transformation. This physical fatigue factor will not be competitive with magnetic storage technology, which can achieve a high number of erase/write cycles. Also, with other materials such as metals and elastomers, this type of medium is highly susceptible to hard bit errors resulting from imperfections in the medium. More particularly, any anomalies or irregularities in the surface of the medium will affect the ability of the top layer to accommodate changes between the two physical states of the bottom layer.